1. Molecular Geometry

2. Molecular geometry is the three-dimensional arrangement of atoms in a molecule. The molecular geometry, or shape, of a molecule is an important factor that affects the physical and chemical properties of a compound. Those properties include melting and boiling points, solubility, density, and the types of chemical reactions that a compound undergoes. In this lesson, you will learn a technique to predict molecular geometry based on a molecule’s Lewis electron dot structure.

3. VSEPR Theory
The valence shell is the outermost occupied shell of electrons in an atom. This shell holds the valence electrons, which are the electrons that are involved in bonding and shown in a Lewis structure. Valence-shell electron pair repulsion theory, or VSEPR theory, states that a molecule will adjust its shape so that the valence electron pairs stay as far apart from each other as possible. This makes sense, based on the fact that negatively charged electrons repel one another. We will systematically classify molecules according to the number of bonding pairs of electrons and the number of nonbonding or lone pairs around the central atom. For the purposes of the VSEPR model, a double or triple bond is no different in terms of repulsion than a single bond. We will begin by examining molecules in which the central atom does not have any lone pairs.

4. Central Atom with No Lone Pairs
In order to easily understand the types of molecules possible, we will use a simple system to identify the parts of any molecule.
A = central atom in a molecule
B = atoms surrounding the central atom
Subscripts after the B will denote the number of B atoms that are bonded to the central A atom. For example, AB4 is a molecule with a central atom surrounded by four covalently bonded atoms. Again, it does not matter if those bonds are single, double, or triple bonds.

5. AB2
Beryllium hydride (BeH2) consists of a central beryllium atom with two single bonds to hydrogen atoms. Note that it violates the octet rule, because the central atom has only 4 valence electrons. This is acceptable because beryllium only has two valence electrons to begin with, so it is not possible for it to create more than two covalent bonds with hydrogen atoms.
According to the requirement that electron pairs maximize their distance from one another, the two bonding pairs in the BeH2 molecules will arrange themselves on directly opposite sides of the central Be atom. The resulting geometry is a linear molecule, shown in a “ball and stick” model. The H-Be-H bond angle is 180° because of its linear geometry.

6. Carbon dioxide is another example of a molecule which falls under the AB2 category. Its Lewis structure consists of double bonds between the central carbon atom and each oxygen atom.
The repulsion between the two double bonds on either side of the carbon atom is no different than the repulsion between the two single bonds on either side of the beryllium in the previous example. Therefore carbon dioxide is also linear, as this achieves the maximum distance between the electron pair bonds.

7. AB3
Boron trifluoride (BF3) consists of a central boron atom with three single bonds to fluorine atoms. The boron atom is an exception to the octet rule, and generally only needs 6 atoms to be stable in a bonded molecule.
The geometry of the BF3 molecule is called trigonal planar. The fluorine atoms are positioned at the vertices of an equilateral triangle. The F-B-F angle is 120°, and all four atoms lie in the same plane.

8. AB4
Methane (CH4) is an organic compound that is the primary component of natural gas. Its structure consists of a central carbon atom with four single bonds to hydrogen atoms.
In order to maximize their distance from one another, the four groups of bonding electrons do not lie in the same plane. Instead, each of the hydrogen atoms lies at the corners of a geometrical shape called a tetrahedron. The carbon atom is at the center of the tetrahedron. Each face of a tetrahedron is an equilateral triangle.

9. The molecular geometry of the methane molecule is referred to as tetrahedral. The H-C-H bond angles are 109.5°, which is larger than the 90° that they would be if the molecule was planar. This way, the bonds are as far apart as possible to minimize electron repulsion. When drawing a structural formula for a molecule such as methane, it is advantageous to be able to indicate the three-dimensional character of its shape. The structural formula is called a perspective drawing. The dotted line bond should be visualized as going back into the page, while the solid triangle bond should be visualized as coming out of the page.

10. AB5
The central phosphorus atom in a molecule of phosphorus pentachloride (PCl5) has ten electrons surrounding it, exceeding the octet rule. This is allowed because phosphorus is a third period element and has access to d orbitals, which will be discussed later on.
Unlike the other basic shapes, the five chlorine atoms in this arrangement are not equivalent with respect to their geometric relationship to the phosphorus atom. Three of the chlorine atoms lie in a plane, with Cl-P-Cl bond angles of 120°. This portion of the molecule is essentially the same as a trigonal planar arrangement. These chlorine atoms are referred to as the equatorial atoms because they are arranged around the center of the molecule. The other two chlorine atoms are oriented exactly perpendicular to the plane formed by the phosphorus atom and the equatorial chlorine atoms. These are called the axial chlorine atoms.

11. In this image, the axial chlorine atoms form a vertical axis with the central phosphorus atom. There is a 90° angle between P-Claxial bonds and P-Clequitorial bonds. The molecular geometry of PCl5 is called trigonal bipyramidal. A surface covering the molecule would take the shape of two three-sided pyramids pointing in opposite directions.

12. AB6
The sulfur atom in sulfur hexafluoride (SF6) also exceeds the octet rule. Unlike the trigonal bipyramidal structure, all of the fluorine atoms in SF6 are equivalent. The molecular geometry is called octahedral, because a surface covering the molecule would have eight sides. All of the F-S-F angles are 90° in an octahedral molecule, with the exception of the fluorine atoms that are directly opposite one another which have a 180° bond angle.

13. Central Atom with One or More Lone Pairs
The molecular geometries of molecules change when the central atom has one or more lone pairs of electrons. The number bonds to the central atom plus the number of lone pairs on the central atom gives us what is called the electron domain geometry. Electron domain geometries refer to the five molecular shapes learned so far: linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral. However, if one or more of the bonding pairs of electrons is replaced with a lone pair, the shape of the molecules is altered. This can apply to any of the geometries discussed above, but for now we will focus on the tetrahedral electron domain geometry.

14. Ammonia
The ammonia molecule contains three single bonds and one lone pair on the central nitrogen atom. The domain geometry for a molecule with four electron pairs is tetrahedral, as was seen with CH4. In the ammonia molecule, one of the electron pairs is a lone pair rather than a bonding pair. The molecular geometry of NH3 is called trigonal pyramidal.

15. Recall that the bond angles in the tetrahedral CH4 molecule are all equal to 109.5°. One might expect the H-N-H bond angles in ammonia to be 109.5° as well, but slight adjustments need to be made for the presence of lone pairs. Within the context of the VSEPR model, lone pairs of electrons are considered to be slightly more repulsive than bonding pairs of electrons, due to their closer proximity to the central atom. In other words, lone pairs “take up more space”. Therefore the H-N-H angle is slightly less than 109.5°. Its actual value is approximately 107°.

16. Water
A water molecule consists of two bonding pairs and two lone pairs. The water molecule, like the ammonia and methane molecules, has a tetrahedral domain geometry. In the water molecule, two of the electron pairs are lone pairs rather than bonding pairs. The molecular geometry of the water molecule is referred to as bent. The H-O-H bond angle is 104.5°, which is smaller than the bond angle in NH3.

17. Summary of VSEPR
The VSEPR model can be applied to predict the molecular geometry of a given molecular compound. There are a number of additional shapes that can be constructed starting from other electron domain geometries and replacing one or more atoms with lone pairs. We will not go over each individual case in this book, but the names for various shapes are provided in the tables below as a reference. To determine the shape of a given molecule, use the following steps:
•Draw the Lewis electron dot structure for the molecule.
•Count the total number of electron pairs around the central atom. This is referred to as the electron domain geometry.
•If there are no lone pairs around the central atom, refer to the next table, to determine the molecular geometry, which is the same as the electron domain geometry.
•If there are one or more lone pairs on the central atom, the molecular geometry (the actual shape of the molecule) will not be the same as the electron domain geometry. Refer to the second table.
•In predicting bond angles, remember that a lone pair takes up more space than a bonding pair or pairs of electrons.

18. Geometries of Molecules in Which the Central Atom Has No Lone Pairs
Atoms Around Central
Atom
Electron Domain Geome-
try
Molecular Geometry
Example 2
linear
BeCl2 3
trigonal planar
BF3 4
tetrahedral
CH4 5
trigonal bipyramidal
PCl5 6
octahedral
SF6

19. Geometries of Molecules in Which the Central Atom Has One or More Lone Pairs

20. Lesson Summary
•Valence shell electron pair repulsion (VSEPR) theory is a technique for predicting the molecular geometry of a molecule. A molecule’s shape provides important information that can be used to understand its chemical and physical properties.
•According to VSEPR, covalent bonds and lone pairs distribute themselves around a central atom in such a way as to maximize their distance from each other.
•Electron domain geometries are based on the total number of bonds and lone pairs, while molecular geometries look only at the arrangements of atoms and bonding pairs.